1. Field of the Invention
This invention relates to an ophthalmological diagnosis method and apparatus, and more particularly to an ophthalmological diagnosis method and apparatus in which the eye fundus is illuminated by a beam of laser light having a predetermined diameter, and scattered laser light reflected from tissue in the eye fundus is evaluated to measure the blood flow state for ophthalmological diagnosis.
2. Description of the Prior Art
Conventional methods that employ laser light to measure the state of the blood flow in the eye fundus include those disclosed in Japanese Patent Laid-open Publications Nos. 55(1980)-75668, 55(1980)-75669, 55(1980)-75670, 56(1981)-125033 and 58(1983)-118730. All of these are methods for determining blood flow velocity based on the laser Doppler effect, so in each case it is therefore necessary to detect the frequency shift of the laser light caused by the Doppler effect. This can be done using either of two arrangements. One comprises splitting the incident laser beam into two beams forming equal angles with respect to the optical axis of the incident laser beam and directing the split beams into the eye to be examined so that they intersect precisely at the position of the eye fundus blood vessel concerned. The other arrangement is to detect laser light scattered by the eye fundus blood cells from two different directions. In both cases the optical system is complex and needs to be high- precision. In addition, the fact that the angle of beam incidence or light detection has to be known in advance makes these methods extremely difficult to apply clinically because of dependency of the eyes to be examined upon patients and impairs the repeatability and reliability of the results thereby obtained. This shows that the laser Doppler method is very useful for application in the industrial field having stable and steady objects because of its precise and sensitive features, but is apt to be influenced by various factors and disadvantageously reduces the repeatability of the results obtained, particularly in the medical field in which biological organisms living in unstable atmospheres and conditions are to be examined.
Further, in actual measurements the results are not obtained as a single Doppler shift frequency but consists of wide-ranging frequency components extending from the low to high frequency region, making it difficult to obtain a reliable absolute velocity value.
Other problems arise from the fact that the laser beam can be directed onto the eye fundus only along path that are perpendicular or nearly perpendicular to the eye fundus. At such angles, the Doppler shift is very small and the beat signals are hard to detect. This is because the laser Doppler method requires the detection of a single beat component. Thus in applications relating to biological tissues, which produce a wide range of irregular interferences, it is preferable to make use of the laser speckle method, the very essence of which is the interference effect of irregularly scattered light.
It is known that when a laser beam strikes an object which causes diffusion or scattering of the beam, the light scattered from the object generally gives rise to a speckle pattern caused by interference between reflected rays of the coherent light. In this case, any movement of the object causing the scattering will cause motion of the speckle pattern which can be detected as a time-course change in light intensity at an observation point. Thus, if the changes in intensity are converted into a signal, it becomes possible to measure the movement of the light-scattering object from the signals. The present invention applies this principle to the measurement of the state of blood flow in living tissue such as, for example, the tissue constituting the eye fundus.
Japanese Patent Laid-open Publications Nos. 60(1985)199430, 60(1985)-203235 and 60(1985)-203236, for example, disclose an application of such speckle phenomena to the measurement of the blood flow. These methods, however, intend to apply in the measurement on the skin surface and thus are inapplicable to the measurement of the blood flow in the eye funds in view of the facts of radiation of a laser beam in a certain intensity and the necessity of a corresponding detection optical system.
For this reason, the inventors have already filed an application for the invention entitled ophthalmological diagnosis method and apparatus (corresponding to U.S. Pat. No. 4,743,107) in which a laser speckle pattern is used to measure the blood flow in the eye fundus. In this method, however, a region of the eye is illuminated with a laser beam having a predetermined diameter greater than that of one blood vessel in the eye, and light scattered from a plurality of blood vessels within the illuminated region of the eye is detected at the Fraunhofer diffraction plane at which the scattered light is superimposed to produce a speckle pattern whose motion is detected, thus improving the stability and repeatability of the measurement obtained. Thus, this method is advantageous because its arrangement enables an overall, average evaluation of the state of blood flow in a plurality of blood vessels included within the irradiated region of the eye, but is impractical when the velocity of blood flow in a single specific blood vessel within the irradiated region is to be measured.
To overcome this drawback, the same inventors proposed an improved ophthalmological diagnosis apparatus using a laser speckle method which makes use of a new detection system so as to be able to evaluate the blood flow velocity of a specified blood vessel. This apparatus is disclosed, for example, in Japanese Patent Laid-open Publications Nos. 63(1988)-242220 and 63(1988)-242221. This, however, disadvantageously requires a detection aperture (for example, pin hole or slit) which must be set on a blood vessel image to be measured at a magnified image plane in order to select one of the specified blood vessels. Further, the detection plane is at an image plane and conjugate with the eye fundus, thus causing the displacement of the image position as the portion of the eye fundus to be measured displaces. This necessitates means for observing the eye fundus image by naked eye for alignment. For this purpose, an observing eyepiece is provided with an indicating mark, which is aligned within its view field to the position of a blood vessel concerned to cause the detection aperture to displace by a mechanical interlocking mechanism in response to the adjustment of the indicating mark for alignment into the position of the corresponding blood vessel image at the magnified image plane. Thus it has been found that the interlocking mechanism is complicated with the total apparatus cost increased, and the mechanical adjustment at the manufacturing between the indicating mark and the detection aperture is also sophisticated. The mechanical interlocking mechanism further includes a mechanical play, causing position setting errors and a poor operational responsibility. Furthermore, there is the necessity of a two-stepped operation to specify blood vessels concerned. One is to carry out positional alignment with the aid of an eye fixation target to illuminate a region including the blood vessels with the laser beam. And the other is to specify one of the blood vessels with the aid of the indicating mark on the eyepiece. This disadvantageously causes a detected position to deviate during the period of the above-mentioned alignment because of the movement of the patient's eye, thus needing renewed alignment or adjustment.
On the other hand, the laser beam is projected on a region of the eye fundus extending over an area greater than the diameter of the blood vessel. This produces light which is scattered from the surrounding tissue outside the blood vessels within the illuminated region of the eye fundus and is greater in intensity than light scattered from the blood flow cells flowing in the blood vessel, thereby making it difficult to clearly discriminate the blood vessel and the surrounding tissue at the magnified image plane. To overcome this drawback, a filtering at the spatial frequency plane is proposed, but this also disadvantageously causes the optical system to be complicated and the quantity of detected light to be reduced greatly.
Furthermore, a speckle pattern of sufficient intensity can not be detected because the eye fundus has too low reflectivity and because observation and photography optical systems used in an eye fundus camera have a large F-number and this makes detected light intensity too small. However, a too strong laser beam cannot be projected onto the eye fundus from the point of view of safety. Thus, a photon correlation method useful to detect light of a very weak intensity has been proposed as shown in Japanese Patent Laid-open Specifications Nos. 62(1987)-275431 (corresponding to U.S.P. 4,743,107) and 63(1988)-242220.
This method is, however, impractical in view of the fact that a detection plate having an aperture sufficiently smaller than the average size of the individual speckles must be set at the detection plane so as to be able to detect the changing light intensity distribution of the speckle pattern sharply. This inevitably causes the reduction of the light detected and necessitates a measurement time (so long as ten to several tens seconds) to obtain sufficiently converged and stabilized photon correlation data. For this reason, the quantity of light projected on the patient's eye increases. The patient should further be under heavy burdens that he must be stationary during measurement. This actually causes the eye movement, thus making the measurement incorrect.
On the other hand, a method has been proposed in which the diameter of the detection aperture is made greater to increase the light detected. This, however, causes an increase in DC component whose rate is much greater than the increase rate of the effective signal components, thus resulting in undesired reduction of an S/N ratio and poor converging stability of photon correlation data.